A cryogenic vessel is a container for fluids at very low temperatures. A typical cryogenic vessel is a double-walled vessel with an inner product container and an outer jacket, with the space between the container and the jacket evacuated and possibly containing thermally insulating and reflective materials.
One significant source of heat leakage into the product container from the outside can be the mechanical support system for supporting the product container within the outer jacket. While support systems are designed to provide minimum heat transfer, there are various mechanical factors that influence the design of the supports, as well, which often are at variance with the desired thermal characteristics.
The structural interrelationship between a support and the product container must reflect the fact that the product container walls are designed to be the thinnest allowable by the applicable pressure vessel codes. This is dictated by the high cost of materials suitable for use in a cryogenic environment and the need to minimize the thermal mass of the product container to avoid excessive cool down losses.
Another factor that must be taken into account is the fact that most metals will undergo a significant thermal shrinkage when cooled from shop fabricating temperatures in the order of 70.degree. F. (21.degree. C.) to super low cryogenic temperatures. The support systems generally incorporate a minimum of two individual components, one located near the service piping end of the vessel to anchor that end of the product container to the outer jacket to minimize piping strains from thermal movements. The other component is located towards the opposite end to allow free linear motion of the product container relative to the outer jacket. The design of the vessel supports must also accommodate radial shrinkage of the product container, while maintaining concentricity of the product container within the outer jacket, and at all times maintaining a solid, vibration free connection between the product container and the outer jacket.
The support system for the product container must also support the dead load of the laden product container. Cryogenic liquids have a specific gravity approaching that water. Thus, a 10,000 gallon (approximately 45,000 litre) vessel of cryogenic liquid might well approach 100,000 pounds (approximately 45,000 kilo-grams) of liquid lading.
Support materials in contact with the cryogenic vessel must necessarily be limited to those with suitable low temperature ductility properties. These include, for example, high nickel steel, some grades of stainless steel, aluminum, copper and some grades of bronze. Some synthetic materials, for example phenolics, teflon, nylon, etc., are usable in compression, but generally not in tension. Materials suitable for use in this environment are referred to herein as cryogenically acceptable materials.
Thermal mass is another important consideration in the design of a cryogenic vessel support system. However, minimizing the stress levels and providing adequately for thermal motion requires the maximizing of load bearing distribution areas in order to minimize load concentrations. This is contrary to the objective of minimum thermal mass, and a reasonable balance must be struck between the two requirements in a satisfactory design.
Since the maximum strength of a cylindrical vessel is attained at the head and shell juncture, supporting the heads will support the vessel if the vessel length is sufficiently short. If the vessel becomes too long, the compressive loads on the top of the cylindrical side wall will cause that part of the vessel wall to buckle. Consequently, some cryogenic vessels will require support between the heads of the product container shell and load distribution on the relatively thin shell becomes a major design consideration.
With the foregoing design criteria in mind, the present invention aims at the provision of a novel support for supporting a product container within an outer jacket of a cryogenic vessel.